1. Field of the Invention
The present invention relates to a method for downhole cementing and, more particularly, to a method for forming a seal or plug of cement in a pipe disposed in an earth borehole, such as an oil well gas, gas well, water well, or the like.
2. Description of the Prior Art
It is well known in the oil well drilling and production arts to use cement for various well operations, such as, for example, to seal off a certain formation below a production packer so that other producing zones can be perforated. In still other cases, to maintain peak production and/or manage the reservoir, oil and gas wells are commonly subjected to maintenance operations referred to in the industry as "workovers." It is common in workovers to emplace temporary or permanent plugs at strategic places in the well to isolate specific areas of the well for servicing. Commonly, this involves the use of mechanical devices known as bridge plugs or retainers that are wedged or mechanically positioned into position into the pipe, e.g., casing or tubing, to form a seal that prevents the flow of fluids below the plug from traveling up the pipe and, conversely, prevents fluids above the pipe from flowing deeper into the well. In most cases, to obtain the required degree of sealing, a cement slurry is placed on top of the bridge plug or retainer, although in certain cases, the cement slurry can be deposited in the well at the desired location, the seal or plug being formed upon expansion and setting of the cement without the use of a mechanical bridge plug, retainer, or the like.
Regardless of whether a bridge plug, retainer, or the like is employed, in workover or similar operations where it is desired to form a cement seal or plug in a pipe, such as tubing or casing, the cement slurry is delivered to the desired location in the pipe by means of a dump bailer, which is well know in the art. Examples of suitable dump bailers are shown in U.S. Pat. Nos. 4,739,829; 4,696,343; 3,872,925; and 5,033,549, all of which are incorporated herein by reference for all purposes.
One problem encountered in the use of dump bailers involves the nature of the cement slurry carried by the dump bailer. Severe problems, including complete job failure or destruction of the dump bailer can develop if the cement slurry becomes too viscous to be expelled from the bailer, or if the slurry does not stay uniform and undergoes settling as the slurry is transferred from the wellhead down the pipe. Still other problems can occur if the cement slurry is incompatible and becomes dispersed with the host fluid in the well, such as, for example, a heavy brine such as a zinc bromide brine. Such incompatibility between the cement slurry and the host fluid can result in gellation of the slurry, preventing the total volume in the bailer from dumping. In this case, the slurry remaining in the bailer would set, rendering the dump bailer useless. Additionally, this intermingling of the cement slurry and an inhospitable host fluid can reduce the shear bond strength of the seal or plug, resulting in seal failure; a longer setting time, increasing the chances of failure to seal; or the inability of the slurry to set at all. Still other problems may occur if the host fluid and cement slurry intermingle and the resulting mixture has a specific gravity that is less than that of the host fluid, which typically results in the resulting mixture containing its cement slurry mass floating up the pipe. All of the above adverse circumstances can result in complete job failure.
In attempts to overcome the above problems, cement slurries are tailored with various chemicals that viscosify and prevent settling, even under higher temperatures, which tend to thin the slurry. Additionally, in certain cases other chemical agents to retard or prevent premature setting are employed. In general, these tailored slurries are often very difficult to mix on the surface (requiring as much as 15-20 minutes to form into uniform slurries).
Accordingly, there exists the need for a method of forming a cement plug or seal in a pipe disposed in an earth borehole that can employ a dump bailer and that utilizes a cement slurry that overcomes many of the prior art problems discussed above with respect to the use of dump bailers and methods for forming cement plugs or seals. In particular, there exists a need for a cement slurry that can be used with a dump bailer cementing technique wherein the cement slurry is compatible with the host fluid.